The present invention relates to a fuel system for supplying pressurised low viscosity fuel to an internal combustion engine. The fuel system comprises a low pressure fuel system, a high-pressure fuel pump, a common rail, a fuel injector, and an engine management system (EMS). The high-pressure fuel pump is arranged to supply pressurised fuel to the common rail, and the common rail is arranged to supply high-pressure fuel to the fuel injector, which is configured to inject high-pressure fuel into a combustion chamber of the combustion engine. The present invention also relates to a corresponding method for providing a recirculating cooling fuel flow through at least a high-pressure fuel pump.
Fuel systems for supplying high-pressure fuel to fuel injectors are well-known in the background art. These fuel systems normally comprise a low pressure fuel system, a high-pressure fuel pump, a common rail with fuel injectors connected thereto. A low-pressure feed pump of the low pressure fuel system feeds fuel to the high-pressure fuel pump, which is configured to supply pressurised fuel to the common rail. Fuel injectors are configured to receive high-pressure fuel from the common rail, and to inject high-pressure fuel into the combustion chamber of the combustion engine.
One potential problem with such fuel systems is fuel boiling in hot conditions, in particular upon use of low viscosity fuels, such as dimethyl ether (DME) or the like. Upon fuel boiling within the high-pressure fuel pump, the volumetric efficiency is reduced, which in turn may lead to reduced or complete loss of fuel pressure. As a result, the engine may stall.
EP1180595 discloses a fuel supply arrangement where a valve 30 is arranged downstream of a low pressure pump 6 and upstream of a high pressure pump 12. When the temperature in the arrangement is increased the valve 30 is closed. The closing of the valve 30 increases the flow and cooling through the second high pressure fuel pump 12.
DE102005012997 discloses a method for reducing pressure in the high pressure area of an injection system, wherein an actuator of an injector is controlled by a control valve and an injector in such a manner that the injector remains closed while the control valve is at least partially opened, thereby allowing fuel to run off from the high pressure area of the injection system via the control valve.
One known measure for avoiding fuel boiling within the high-pressure fuel pump is to provide the high-pressure fuel pump with a cooling orifice situated upstream of an inlet metering valve of the high-pressure fuel pump. Such a solution is known from US 2010/0282211 A1. The cooling orifice provides a flow path from a high-pressure fuel pump inlet to a low pressure pump outlet, and the flow of fuel through the high-pressure fuel pump acts to cool the pump by conducting away heat generated therein during operation. Fuel boiling within the high-pressure fuel pump can however occur during certain operating conditions and fuel pump designs, despite the provision of a cooling orifice.
There is thus a need for an improved fuel system removing the above mentioned disadvantage.
It is desirable to provide an inventive fuel system, wherein the risk for fuel boiling within the high-pressure fuel pump is reduced.
The invention concerns, according to an aspect thereof, a fuel system for supplying pressurised low viscosity fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine, in particular a compression ignition engine, said fuel system comprising a low pressure fuel system, a high-pressure fuel pump, a common rail, at least one fuel injector, and an engine management system (EMS), wherein said high-pressure fuel pump is arranged to supply pressurised fuel to said common rail, and said common rail is arranged to supply high-pressure fuel to said at least one fuel injector, which is configured to inject high-pressure fuel into a combustion chamber of said combustion engine.
The inventive fuel system is characterised in that said engine management system (EMS) may initiate a recirculating cooling fuel flow through at least the high-pressure fuel pump for avoiding fuel boiling by means of either increasing the target pressure of the fuel within the common rail above a threshold level, which triggers opening a high-pressure fuel relief valve that is arranged downstream of said high-pressure fuel pump, such that at least part of the fuel supplied by said high-pressure fuel pump is returned to said low pressure fuel system via said high-pressure fuel relief valve, or providing increased internal fuel leakage within said at least one fuel injector, such that at least part of the fuel supplied by said high-pressure fuel pump is returned to said low pressure fuel system by a return line.
The invention further concerns a method for providing a recirculating cooling fuel flow through at least a high-pressure fuel pump, wherein said fuel pump is part of a fuel system that is configured to supply pressurised low viscosity fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine, in particular a compression ignition engine, said fuel system comprising a low pressure fuel system, a high-pressure fuel pump, a common rail, at least one fuel injector, and an engine management system (EMS), wherein said high-pressure fuel pump is arranged to supply pressurised fuel to said common rail, and wherein said common rail is arranged to supply high-pressure fuel to said at least one fuel injector, which is configured to inject high-pressure fuel into a combustion chamber of said combustion engine.
The inventive method being characterised by initiating a recirculating cooling fuel flow through at least the high-pressure fuel pump for avoiding fuel boiling by means of either increasing the target pressure of the fuel within the common rail above a threshold level that triggers opening a high-pressure fuel relief valve that is arranged downstream of said high-pressure fuel pump, such that at least part of the fuel supplied by said high-pressure fuel pump is returned to said low pressure fuel system via said high-pressure fuel relief valve, or providing increased internal fuel leakage within said at least one fuel injector, such that at least part of the fuel supplied by said high-pressure fuel pump is returned to said low pressure fuel system by a return line.
The inventive fuel system and corresponding method reduces the probability of fuel boiling within the high-pressure fuel pump by guaranteeing a fuel cooling flow throughout the entire high-pressure fuel pump. The prior art solution with a cooling orifice as described above only cools a part of the high pressure fuel pump, namely the part up to the cooling orifice itself, but not the part beyond the inlet metering valve of the high pressure fuel pump. Fuel vapour bubbles developing downstream of the inlet metering valve, i.e. at the suction side of the high pressure pumping unit of the high pressure fuel pump, will thus not be evacuated by the prior art solution, thereby drastically reducing the pump volumetric efficiency.
According to a first embodiment of the invention for guaranteeing said fuel cooling flow throughout the entire high-pressure fuel pump, a cooling fuel flow from the high pressure fuel pump to the low pressure fuel system is provided via a high pressure fuel relief valve that is arranged downstream of said high-pressure fuel pump.
Cooling fuel flow is thereby guaranteed to pass all essential parts of the high pressure fuel pump, thereby suppressing the formation of and evacuating any unwanted fuel vapour bubbles not only upstream the inlet metering valve, but also downstream said inlet metering valve, i.e. at the high pressure pumping unit. Said cooling flow via said high pressure fuel relief valve is provided by temporarily increasing the target pressure of the fuel within the common rail above a threshold level, which triggers opening a high-pressure fuel relief valve.
In case an existing safety relief valve is provided downstream the high pressure fuel pump for preventing damages to the high pressure pump, common rail, or fuel injectors due to excessive fuel pressure, then said existing safety relief valve may preferably be used as high pressure fuel relief valve, such that no additional high pressure fuel relief valve is required, thereby reducing cost of the fuel system, as well as increasing reliability and durability of the fuel system.
According to a second embodiment of the invention for guaranteeing said fuel cooling flow throughout the entire high-pressure fuel pump, a cooling fuel flow from the high pressure fuel pump to the low pressure fuel system is provided by increased internal fuel leakage within said at least one fuel injector. After being leaked from the injector, the cooling fuel flow is returned to said low pressure fuel system by a return line, which connects each fuel injector with the low pressure fuel system. This solution does normally not require any additional hardware components, and is preferably implemented merely by new software. No, or at least no additional high pressure fuel relief valve is consequently required, thereby reducing cost and increasing reliability and durability of the fuel system.
According to the invention, said engine management system (EMS) is preferably arranged to, upon determining a risk of fuel boiling within said high-pressure fuel pump, initiate said recirculating cooling fuel flow through said high-pressure fuel pump. The recirculating cooling fuel flow is thus only initiated when a risk of fuel boiling is determined. When no or only a low risk of fuel boiling is estimated, no recirculating cooling fuel flow is provided. The level of risk is preferably determined by the engine management system based on one or more indicators, as discussed more in detail below. The degree of recirculating fuel flow may be fixed or variable.
According to the invention, said engine management system (EMS) is preferably configured to determine that there is a risk of fuel boiling within said high-pressure fuel pump when the engine is operated in a fuel non-injection mode. Engine operation in a fuel non-injection mode is an easy to implement indicator for an elevated risk of fuel boiling, because during fuel non-injection mode, essentially no fuel flows through the complete high pressure pump, i.e. also passing the high pressure pumping unit. The high pressure fuel pump inlet metering valve is nearly closed, and then the fuel within the high pressure fuel pump may quickly vaporise, leading to loss of volumetric efficiency. Fuel non-injection mode may for example occur during coasting or engine braking of a vehicle.
According to the invention, said engine management system (EMS) is preferably configured to determine the risk of fuel boiling within said high-pressure fuel pump based on at least one of the following parameters: engine operation mode, duration of said engine operation mode, fuel temperature adjacent and/or within said high-pressure fuel pump, fuel pressure adjacent and/or within said high-pressure fuel pump, fuel boiling point. As described above, fuel non-injection mode may be used as a more simple indicator for elevated risk of fuel boiling. However, in certain circumstances, it may be advantageous not to initiate recirculating cooling fuel flow based merely on entering a non-injection mode. For example, the duration of the engine operation mode is relevant because a short time period of engine non-injection mode does not immediately result in fuel boiling. Moreover, the fuel temperature and the fuel properties itself are relevant indicators that may be taken into account upon determining the risk.
According to the invention, said high-pressure fuel relief valve is preferably a mechanical relief valve, which preferably is arranged along the fuel supply line between said high-pressure fuel pump and said common rail, or connected to said common rail. A mechanical relief valve implies low cost, not only for the valve itself but also because no electronic control thereof is required. The positioning of the valve is somewhere downstream from the high pressure fuel pump.
According to the invention, said high-pressure fuel relief valve preferably also functions as a safety pressure limiting relief valve of said fuel system for preventing damages to any of said common rail, said at least one fuel injector, or said high-pressure fuel pump due to excessive fuel pressure. By providing the high-pressure fuel relief valve with the dual functionality of allowing recirculating cooling fuel flow, as well as operating as safety pressure limiting relief valve, only a single relief valve is required downstream the high pressure fuel pump, thereby reducing cost and increasing reliability and durability of the fuel system.
According to the invention, said high-pressure fuel relief valve is preferably a single relief valve downstream of said high-pressure fuel pump and upstream of said at least one fuel injector.
According to the invention, said fuel system could further comprise an additional safety relief valve arranged downstream of said high-pressure fuel pump and upstream of said at least one fuel injector, wherein the threshold level that triggers opening said high-pressure fuel relief valve is lower than the threshold level that triggers opening of said additional safety relief valve. This arrangement comprising two relief valves, each having a different threshold for triggering opening thereof, may be advantageous in terms of safety aspects of the fuel system due to relief valve redundancy. Moreover, the additional safety relief valve may be electronically controlled, such that the threshold for triggering opening thereof may vary depending on the operating mode, and the like.
According to the invention, said temporarily increased internal fuel leakage within said at least one fuel injector is preferably provided by increasing valve control leakage within said at least one fuel injector. This type of temporarily increased internal fuel leakage is easily implemented, preferably by suitable software only. No amendments of the high pressure fuel pump or common rail is necessary, thereby avoiding expensive redesign.
According to the invention, said valve control leakage within said at least one fuel injector is preferably increased by allowing inlet of fuel from said common rail to an internal injector volume of said at least one fuel injector, while simultaneously and/or subsequently allowing discharge of fuel from said internal injector volume to said return line, wherein said inlet and discharge of fuel is directly or indirectly controlled by said engine management system (EMS) such that no fuel is injected into said combustion chamber.
According to the invention, said at least one fuel injector preferably comprises: a spring-loaded nozzle for injecting high-pressure fuel into said combustion chamber; an inlet valve arranged on a fuel supply line connecting said nozzle with said common rail, which inlet valve is directly or indirectly controlled by said engine management system (EMS); and a fuel spill valve arranged on a fuel return line connecting said low pressure fuel system with said fuel supply line between said inlet valve and said nozzle, which fuel spill valve is directly or indirectly controlled by said engine management system (EMS); wherein said inlet of fuel is controlled by said inlet valve, and said discharge of fuel is controlled by said spill valve, and wherein during the time of temporarily increased internal fuel leakage within said at least one fuel injector said inlet valve and spill valve are controlled such that the fuel pressure within said internal injector volume is not exceeding a threshold level that triggers opening of said nozzle, thereby preventing fuel from being injected into said combustion chamber.
According to the invention, said temporarily increased internal fuel leakage within said at least one fuel injector is preferably provided by means of a series of short duration control pulses from said engine management system (EMS) for providing repeated short duration inlet of fuel into said internal injector volume and discharge of fuel from said volume. Each fuel inlet duration must be sufficiently short not to result in injection of fuel into said combustion chamber, which for example depending on fuel injector design may occur when the fuel pressures downstream the inlet valve of the fuel injector exceeds the nozzle closing force. Hence, a series of short duration control pulses results in a sufficient recirculating cooling fuel flow through the fuel injector and back to the low pressure fuel system via the return line.
According to the invention, said temporarily increased internal fuel leakage within said at least one fuel injector is preferably configured to be realised also during engine injecting operation mode by scheduling said inlet and discharge of fuel between time periods of normal inlet and discharge of fuel associated with said engine injecting operation mode. Thereby, recirculating cooling fuel flow may be provided not only in an engine non-injecting operation mode, but also during an engine injecting operation mode. This may be advantageous especially during low fuel consumption operating modes due to the relatively low cooling effect of the fuel consumption flow.